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2.1 KiB
2.1 KiB
Cross-Layer Thinking Guide
Purpose: Think through data flow across layers before implementing.
The Problem
Most bugs happen at layer boundaries, not within layers.
Common cross-layer bugs:
- API returns format A, frontend expects format B
- Database stores X, service transforms to Y, but loses data
- Multiple layers implement the same logic differently
Before Implementing Cross-Layer Features
Step 1: Map the Data Flow
Draw out how data moves:
Source → Transform → Store → Retrieve → Transform → Display
For each arrow, ask:
- What format is the data in?
- What could go wrong?
- Who is responsible for validation?
Step 2: Identify Boundaries
| Boundary | Common Issues |
|---|---|
| API ↔ Service | Type mismatches, missing fields |
| Service ↔ Database | Format conversions, null handling |
| Backend ↔ Frontend | Serialization, date formats |
| Component ↔ Component | Props shape changes |
Step 3: Define Contracts
For each boundary:
- What is the exact input format?
- What is the exact output format?
- What errors can occur?
Common Cross-Layer Mistakes
Mistake 1: Implicit Format Assumptions
Bad: Assuming date format without checking
Good: Explicit format conversion at boundaries
Mistake 2: Scattered Validation
Bad: Validating the same thing in multiple layers
Good: Validate once at the entry point
Mistake 3: Leaky Abstractions
Bad: Component knows about database schema
Good: Each layer only knows its neighbors
Checklist for Cross-Layer Features
Before implementation:
- Mapped the complete data flow
- Identified all layer boundaries
- Defined format at each boundary
- Decided where validation happens
After implementation:
- Tested with edge cases (null, empty, invalid)
- Verified error handling at each boundary
- Checked data survives round-trip
When to Create Flow Documentation
Create detailed flow docs when:
- Feature spans 3+ layers
- Multiple teams are involved
- Data format is complex
- Feature has caused bugs before